1. Field of the Invention
The invention relates to a body structure for vehicles, and, more particularly, to a vehicle body structure having a frame.
2. Description of Related Art
Typically, In a known vehicle body structure, a front frame is constructed as a boxed channel as described in, for example, Japanese Unexamined Patent Publication No. 8-324454. This front frame is constructed of a pair of flanged U-channel members with their flanges welded together. The front frame has a front crumple zone formed with transverse beads arranged in a lengthwise direction from the front to the back of the frame which is programmed to collapse and absorb the energy of an impact in the event of a frontal collision.
In general, in the event where an external impact acts on a frame at the front end or the rear end in the lengthwise direction, the frame starts to collapse initially with an impact which is not so great and absorbs the energy of the impact to ease the passenger's feeling of an impact and then continuously and regularly collapses without bending to absorb the significantly strong energy of the impact before it reaches the passengers. In order for the frame to continuously and regularly collapse, if having a rectangular cross-section, the frame is typically programmed such that all four walls collapse bellows-fashion at regular pitches peculiar thereto in the lengthwise direction and each adjacent walls are deformable in different shapes, namely concave and convex, as viewed in a same cross-sectional plane.
Conventional type of front frame, to which a front bumper is attached, is formed with a variety of beads so as to collapse in a programmed fashion in the event where an impact acts on the frame through the bumper upon an occurrence of a frontal collision. As is described in, for example, Japanese Unexamined Patent Publications Nos. 61-287871, 5-305877 and Japanese Unexamined Utility Model Publication No. 2-24777, a frame has a variety of beads arranged in parallel in the lengthwise direction programmed to cause lengthwise collapse of the frame with maintaining a relatively large yield strength. Further, as is described in, for example, Japanese Unexamined Patent Publication No. 4-231268, a frame is constructed of a boxed channel member, each adjacent walls being formed with different beads, such as concave and convex, as viewed in a same cross-sectional plane. This type of front frame collapses initially at the beads in the event of a frontal collision so as to control and lower an initial maximum yield strength. In addition, as is described in, for example, Japanese Unexamined Patent Publication No. 8-324454, a frame is formed with beads contributory to maintaining a relatively large yield strength during a collapse and beads contributory to lowering the initial maximum yield strength.
In order to provide a rigid structural foundation against frontal collision for a vehicle body and to provide a solid anchorage for an engine and a suspension system, it is desirable for a front frame having high cross-sectional stiffness to have a rectangular cross-section, as described in, for example, U.S. Pat. No. 3,912,295. A box-shaped front frame of some sort constructed of a pair of flanged U-channel members with their flanges welded together, the frame has a rectangular cross section uniform almost over the entire length and the flanges are transversely off in the lengthwise direction, as described in, for example, U.S. Pat. No. 4,133,549. The prior art front frame is typically provided with end flanges bent laterally to which a front bumper is secured.
Vehicle body is possibly hit an impact in various directions, for example in a lengthwise direction from front or from behind in the event of a frontal or a rear-end collision, respectively, in a slantwise direction from front or from behind in the event of what is called an offset collision, and in a transverse direction in the event of a lateral collision. In the case where the front body is constructed of a front frame, the front frame is essential to be rigid against impacts acting thereon in different directions. The front frame, however, has somewhat conflict demands for stiffness at different lengthwise areas thereof. For example, because of a solid anchorage for a suspension system, the front frame at its rear area must be stiff against bending due to external force vertically acting thereon through the suspension system and is accordingly essential to have high cross-sectional stiffness, in particular, in the transverse direction. On the other hand, the front frame at its front area must withstand a collision impact acting thereof in various directions. However, if the front frame at its front area causes bending deformation due to a frontal offset collision, it is hard for the front frame at its front area to absorb the energy of a frontal collision impact through contract or collapse at the beads. Accordingly, the front frame at its front area is essential to have high cross-sectional stiffness, in particular, in the vertical direction.
As was previously described, impacts acting on the front bumper during a frontal collision is not always directed in parallel to the front frame. The front bumper secured to the front frame is bent at either one end first by such a slantwise impact in the initial stage of a collision and deflects the impact toward the end of the front frame with an effect of blocking transmission of the impact to the front frame. Consequently, even though the front frame has a variety of beads, it can not only crash as programmed but perform functions such as lowering an initial impact and absorbing the energy of an impact in the event of a collision.
Further, if increasing the cross-sectional area of the front frame to provide a large cross-sectional stiffness, an increase in weight of the vehicle body is produced, which is always against a demand for a lightweight body.